1. Field of the Invention
The present invention relates generally to a memory architecture for computer systems and more particularly to a memory subsystem comprised of internal memory and control for external memory.
2. Discussion of Prior Art
A typical personal computer system has a central processing unit (CPU) with an external main memory and has a graphics display subsystem with its own memory subsystem. Part of this memory subsystem is a frame buffer that provides the output to the display, and part of this subsystem may be used for off-screen operations. However, the graphics display subsystem memory and the main system's pool of memory do not share data efficiently or move data efficiently from one memory subsystem to the other.
Another typical personal computer system has a single memory subsystem for both the CPU and the graphics subsystem. The performance of this type of computer system is lower than that of computer systems that have separate memory subsystems for the graphics display subsystem and for the CPU. Even though these single external memory systems can support a cache memory for the CPU, their overall performance is still lower because the memory bandwidth is shared between the graphics and CPU subsystems. These computer systems are very limited in their ability to achieve good performance for both the CPU and graphics subsystems. In order to be cost effective, these systems typically use a lower cost main memory that is not optimized for the special performance needs of graphics operations.
For systems that use a single external memory subsystem to perform all of their display refresh and drawing operations, performance is compromised by the memory bandwidth for these operations being shared with the memory bandwidth for the CPU. "Refresh" is the general term for taking the information contained in a frame buffer memory and sequentially transferring the information by rows to a palette digital-to-analog converter (DAC) to be displayed on an output device such as a monitor, TV or flat panel display. The frame buffer's entire contents needs to be transferred to the output device continuously for the displayed image to be visible. In the case of a monitor, this refresh is performed typically between 75 and 95 times per second. For high-resolution color systems, the refresh process consumes an appreciable portion of the total bandwidth available from the memory.
In addition to the refresh bandwidth, the graphics subsystem performs drawing operations that also consume an appreciable amount of bandwidth. In the case of 2-D graphics acceleration the drawing operations include Bit-BLt (Bit Block Transfers), line drawing and other operations that use the same common pool of memory.
Intel and other companies in the PC industry have designed an accelerated graphics port (AGP) bus and an associated system architecture for combining graphics and chipsets. AGP is a second private bus between the main memory controller chipset and the graphics display subsystems. AGP and the associated system architecture allow the storage of 3-D texture memory in the main memory that can be accessed by the graphics subsystem. This is one limited use of shared main memory for a graphics function. However, because there is a single bus between the graphics subsystem and the main memory controller chipset, this bus limits the system performance. This single bus is shared by all CPU commands to the graphics controller, any CPU direct reads or writes of display data, all texture fetches from main memory and any other transfers of display information that is generated or received from the CPU or I/O subsystems (i.e. video data from a capture chip or a decoder).
AGP is designed to overcome the above-described performance limitations from using the main memory subsystem for display refresh and drawing operations. AGP systems overcome these limitations by a brute force requirement that the graphics subsystem on the AGP bus have a separate frame buffer memory subsystem for screen refresh and drawing operations. Using frame buffer memory is a good solution for eliminating the performance penalties associated with drawing and refresh operations. Meanwhile, as a frame buffer is always required, AGP systems do not allow for screen refresh to be performed from the main system memory. This does not allow the optimization of refreshing all or part of the screen from main memory.
Additionally, the drawing operations must be performed in the graphics display memory and are therefore performed by the graphics subsystem controller. Also limiting the dedicated frame buffer system flexibility, the graphics subsystem controller can not efficiently draw into the main system memory.
Separating the frame buffer memory from the main system memory duplicates the input/output (I/O) system data. For example, this occurs in a system where video data enters the system over an I/O bus through a system controller and then is stored in the main system memory. If the data is displayed, it needs to be copied into the frame buffer. This creates a second copy of the data, transfer of which requires additional bandwidth.
Another alternative is to have a peripheral bus associated with the graphics controller where the I/O data is transferred to the frame buffer. While this allows display of the data without additional transfers over a system bus, the data remains local to the display subsystem. The CPU or main I/O systems can not access the data without using a system bus. For systems with a shared memory subsystem, the I/O data enters a shared memory region. It is then available to either the display subsystem or the CPU.
FIG. 1 shows a diagram of a standard, prior art memory architecture 100. A CPU subsystem 102 is connected to a subsystem 104 which is connected to an external system Random Access Memory (RAM) 110 and to a peripheral component interface (PCI) bus 112. Subsystem 104 contains a system controller 106 and a graphics controller 108 that is connected to a display (not shown in FIG. 1). The system has a single external memory subsystem 110 for both the graphics display and CPU 102.
FIG. 2 is a diagram of the current state-of-the art personal computer memory architecture 200 having separate memories for the CPU and for the graphics display. A CPU subsystem 204 is connected to a system controller 206 that is connected to an external system RAM 210 and to a PCI bus 216. System controller 206 is also connected through a dedicated AGP bus 214 to a graphics controller 208 that is connected to a graphics RAM 212, which is external or integrated with the controller, and to a display 202. CPU subsystem 204 can not treat graphics RAM 212 as an extension of system RAM 210, and graphics subsystem 208 can not use system memory 210 for display refresh.
What is needed is an integrated system controller that supports a memory architecture which combines internal and external memory in which common memory can be used for display memory and main memory, without having inadequate bandwidth access to the common memory to impair performance.